The just merged Alpine CI run for LibreSSL from #9949 failed since it
missed the changes of the refactoring PR #10369. This change applied the
refactoring for Alpine as well, hopefully making the CI happy.
First, the icinga_legacytimeperiod/dst test was excluded, as it fails on
Alpine most likely due to some differences between musl and glibc. After
some debugging, I disabled the test as the Alpine packages does.
More build dependencies were added from the Alpine package, allowing to
only disable MySQL and PostgreSQL support as these libraries have fixed
dependencies on OpenSSL, conflicting with LibreSSL.
In addition, I have added comments where I was first puzzled.
